Cell killing by various nitrosoureas and the potentiating effect of caffeine

Cell killing by various nitrosoureas and the potentiating effect of caffeine on cell killing were quantified by means of colony-forming assays in a clonal derivative of M3-1 Chinese hamster cells. The dose-response relationships for ethylnitrosourea and 3 chloroethylnitrosourea derivatives were compared with that of 1,3-bis(trans-4-hydroxycyclohexyl)-1-nitrosourea, which is a purely carbamoylating agent. The chloroethylnitrosoureas with both alkylating and carbamoylating activity were the most toxic, followed by a chloroethylnitrosourea with alkylating activity alone and then by the purely carbamoylating agent. The least toxic agent was ethylnitrosourea, which has both alkylating and carbamoylating activity. Caffeine (when present through the entire period of colony growth) potentiated cell killing for all nitrosoureas with alkylating activity but had no influence on cell killing by the purely carbamoylating agent. Because carbamoylation occurs mainly with proteins, whereas nucleic acids as well as proteins are cellular targets for alkylation, it is concluded that potentiation of cell killing by caffeine is based on the reaction of a drug with the cellular DNA.